1. Field of the Invention.
This invention relates to new and improved method and apparatus for the imaging of pulse echoes and is particularly adapted for use in acoustic interrogation systems for non-invasive biophysical diagnosis.
2. Description of the Prior Art.
Although a wide variety of prior art acoustic interrogation systems are available for non-invasive biophysical diagnosis, it may be understood that the same, in general will be found to be lacking in the provision of displays of good axial resolution since such systems are generally limited in operation to several cycles of transducer ring as determined by the transducer Q and the natural resonant frequency of the transducer. This is so since it is known that, for example, two boundaries separated in space by a distance S along the direction of acoustic energy propagation can only be resolved by a pulse envelope or pulse of width T when S= CT/2 where C is the velocity of propagation of the acoustic energy in the ensonified materials. In addition, although detection can be traded off in the operation of the systems of the prior art to provide increased spatial resolution in the brightness mode by raising the detection threshold level, it is believed clear that this can only be accomplished at the expense of the detection of weak echoes to therefore result in incomplete images.
Many of these prior art systems operate through use of trains of relatively uncontrolled sinusoidal energy pulses of substantial width to produce a pulse echo envelope which is representative of the modulation effects of the acoustic boundaries of the material under interrogation, and this envelope is then utilized for conventional Z axis modulation of a CRT or like display device. Envelope detection of this nature is well known by those skilled in this art to be signal processing technique that maximizes the signal to noise ratio at the expense of discarding phase detection. Thus, and although phase information based on relative acoustic impedance ratio information at said acoustic boundaries of the material under interrogation is inherent in the pulse echo signals received by these prior art devices, the same is lost through the process of envelope detection to thus significantly detract from the readability of the provided display. Too, since the prior art devices under discussion effect Z axis modulation as described in proportion to the intensity of the detected echo envelope level which exceeds a predetermined threshold, it is believed clear that such echo levels are a function not only of the impedance ratio of interest at the material boundary, but also of the attenuation of the acoustic energy due to energy absorption, spreading and the like. As a result, the brightness of the provided CRT display in such prior art devices is not representative of said impedance ratios, only, as should be obvious. Further, the use as described of trains of relatively uncontrolled sinusoidal energy pulses of substantial width by these prior art devices renders the detection of phase information from the resultant echo pulses by simple integration impossible. Also, the combination of transducer ringing and increased threshold setting as discussed hereinabove will, in biophysical interrogation applications, almost inevitably produce artifactual tissue boundaries to significant diagnostic disadvantage as should be obvious. In addition, few if any of such prior art devices are known which can automatically apply a correction for the known input and output boundary conditions of the material under interrogation.
A further significant disadvantage of some of the prior art acoustic interrogation systems resides in the fact that the same require the use of the complex mathematical procedure of echo signal deconvolution to determine the impulse response of the medium under interrogation. More specifically, it may be understood that those prior art systems which do require the use of deconvolution will most likely require the use of a relatively complex digital computer in the echo signal processing circuitry, and will be somewhat limited in the types or shapes of input waveforms that can be utilized to insure that the impulse response does not simply oscillate around zero to no useful purpose as should be obvious. Too, the use of deconvolution gives rise to the further problem that small amounts of distortion in the input waveform, as can and will generally result from signal scattering, refraction, diffraction or the like, will increase exponentially during signal processing to result in not insignificant errors in the final impedance ratio calculations.